By day, he is Justin Edwards: a modest and mild-mannered comic, one third of sketch troupe the Consultants. By late afternoon, he is Jeremy Lion: a horribly misjudged kiddies' entertainer, who terrorises infants with his educational puppetry and consumes prodigious volumes of booze.

Last year, Lion's Happy Christmas Show was one of the Fringe's sleeper hits. It starred a snowman who doubled as a biology lesson, and saw Lion ventriloquise a turkey while necking a tin of Special Brew. Be warned if you're considering taking your six-year-old to this year's follow-up, Happy Birthday.

So how did Edwards turn this fairly uninspired concept ("The idea," he admits, "wasn't the most original in the world") into the hottest new character act in comedy? Jeremy Lion first appeared as a sketch three years ago in a show by the Consultants, who won a Perrier best newcomer award in 2002. "It had been done before," says Edwards. "There are drunken clowns, there's Mr Partridge in Hi-de-Hi. So it was about finding a different angle; something that wasn't obvious." He teamed up with George Cockerill, who plays sullen keyboardist Leslie, and launched Jeremy Lion's show on the London fringe.

As the character grew, Edwards learned what made him tick. "He doesn't realise just how inappropriate he is. Also, he can't sing, he can't string things together and he can't operate props. But he's got this absolute self-belief that what he does is right."

In this year's show, Edwards reveals the roots of Lion's certainty: Jeremy is following in the footsteps of his father, Uncle Bungle the clown. "His father did this before him, so there's this cycle of abuse. He's convinced that the children, as they sit there in stunned, disgusted silence, are actually enjoying themselves."

Stunned silence ain't the half of it. Lion's show is like Play School meets Hellraiser. Atop a giant birthday cake, a red-eyed bat shrieks about dental care. There's a puppet called Mr Shush, with nails for teeth, whose job is to keep the kiddies quiet. (It works!) The props, which are fantastic, are made by Edwards' co-writer, James Eldred. "We'll say to him things like, 'What about a giant octopus with detachable tentacles?' And two months later he turns up with this thing, and we're like, 'Christ, what do we do with this?'"

The mollusc in question is Mr Octopus Rhyme, whose tentacles are used to depict the dangers of visiting the beach. That is the funniest aspect of Lion's act: his old-school faith in educating as well as entertaining his young charges. "Last year," says Edwards, "I saw this bloke doing educational songs for children. All of them were really preachy. 'Now we're going to sing a song about the dangers of matches. Don't strike matches, children!' Or, 'Now we're going to sing a song about crossing the road.' Awful stuff. There was no humour behind it at all. I thought: this is brilliant. This is exactly what Jeremy should be doing."

It's good to hear that Edwards is keeping abreast of developments in children's entertainment - although he hasn't seen the US documentary Capturing the Friedmans, which raised the bar for children's clowns with murky hinterlands. Mind you, Edwards has personal experience. "I did a few children's parties when I was younger," he confesses, sheepishly. "I lived in a small village and it was a case of people saying, 'Will you come and do my son's birthday?'" Does he draw on the experience when creating Lion's shows? "A bit, yes." Pause. "Because I was quite shit."

But he'd rather Jeremy Lion was kept as far from real children as possible. "Last year, quite a few came. That was uncomfortable. I thought, if they're under six, they might still believe in Father Christmas, and this won't do them any good at all." Too many sprogs in the crowd, he says, and the adults feel uncomfortable. And so does he. "I would catch sight of these poor children staring up at me while I was disembowelling the snowman. It's not great." His publicity material is plastered with the phrase Not Suitable for Children.

Edwards ascribes some of the character's success to the changing fashions in children's entertainment. Not only is Jeremy Lion a gloriously sick joke, he's also a nostalgic throwback, particularly popular with older audiences. "There's an element of Butlins redcoat to Jeremy," says Edwards. "My parents' generation grew up with children's presenters who were avuncular and old. Even when I was young, the Blue Peter presenters were John Noakes and Peter Purves. Now children's presenters are like older brothers or sisters. They're 17 or 18. In my parents' youth, they were 60-year-old men, improperly dressed up as clowns."

That's the tradition Jeremy Lion belongs to - and Edwards wants to render it as convincingly as possible. With character comedy, credibility is always an issue: why is a children's entertainer performing to adults at the Edinburgh Fringe? Edwards imagines that Lion's hour "is a showcase. He wants to be booked for children's parties." His presence on the bill at a stand-up comedy club, say, would be less easy to justify. Likewise, telly - an avenue for Lion that Edwards is beginning to explore. "Why on earth would this bloke be allowed on television?" asks Edwards. "Would he actually be presenting a children's programme? And if so, how long could it possibly go before they pulled the plug on it?"

In the meantime, he'll settle for TV work with the Consultants, who are already established on Radio 4. With cohorts Neil Edmond and James Rawlings, Edwards is developing a series for the BBC, who, he says, "want everything at the moment to be like Little Britain". Purveyors of sharp wit and wordplay, the Consultants aren't like Little Britain - and nor are they like the League of Gentlemen, another three-man troupe whose shadow looms long over TV sketch comedy. Edwards has something more traditional in mind. "Instead of devising ingenious devices for linking things, we want to say, no, it's just half an hour of sketches. That's what we do."

Lion, in the meantime, will keep peddling his wares at Edinburgh. That is, if Edwards' liver holds out. So did he really swig a glass of wine for each of the 12 Days of Christmas? And is that a real can of Special Brew he sinks in one almighty gulp? "Yes, there's quite a lot of real drinking in there," says Edwards. Last year's show almost knocked him out. "I had to have two or three hours' sleep afterwards, or I'd never be able to do the Consultants in the evening." This year's post-show fug can be dispelled, he says, with "a few cups of tea." And anyway, "I sweat so much in there, I treat the drinking as rehydration." But, over the whole festival, that's a lot of, er, rehydration for one body to cope with. "I will have September off, lying in a darkened room," he says. "But this isn't a character I'll be able to do forever. I'll have to tone it down at some stage or I will kill myself."

A song by Jeremy Lion

I come to pleasure children To make them yell and smile From Arthur's Seat, down Princes Street And up the Royal Mile.

Pied Piper-like I dance through town My coloured robes flow in the breeze See the children follow like happy rats While Leslie plays the keys.

Puppets, magic, songs and balloons That's my entertainment The children will all laugh as they learn With my educational bent.

Scottish joy is in my heart And their cakes are in my larder Embrace I now the Gaelic treats Scotch eggs, Scotch whisky and Scotchguard (er).

And so the clown puts on his face His massive shoes, his ruff 6.50 at the Pleasance daily Come and have a go if you think you're young enough.